Fix for #21456 (Regressions in attribute allocations for non-generic attributes)

[NickCraver]

This is a quick-fix for #21456 where regressions between netcoreapp2.1 and netcoreapp3.0 were discovered on most attribute pathways due to the allocation overhead in the generic-supporting pathways. This was introduced in #9189 (thanks @jkotas for the quick find). The workaround here is to simply not take that slow/expensive path for non-generics.

While I'd like to optimize RuntimeModule.ResolveMethod further, there's a public surface area in play there that makes the changes non-trivial. There, we'll have to choose overhead on the public path (which may still be a net win), or duplication in code for another path.

Benchmarks are available in dotnet/performance - run via:

C:\git\dotnet\performance\src\benchmarks\micro

dotnet run -c Release -f netcoreapp3.0 -- --runtimes netcoreapp3.0 --filter System.Reflection.Attributes.GetCustom*
dotnet run -c Release -f netcoreapp3.0 -- --filter System.Reflection.Attributes.GetCustom* --coreRun "C:\git\dotnet\coreclr\bin\tests\Windows_NT.x64.Release\Tests\Core_Root\CoreRun.exe"

Before (netcoreapp3.0 preview 1 - same as master)

BenchmarkDotNet=v0.11.3.886-nightly, OS=Windows 10.0.17763.134 (1809/October2018Update/Redstone5)
Intel Core i7-7700HQ CPU 2.80GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.0.100-preview-009820
  [Host]     : .NET Core 3.0.0-preview-27205-02 (CoreCLR 4.6.27204.02, CoreFX 4.7.18.60501), 64bit RyuJIT
  Job-JKCWXL : .NET Core 3.0.0-preview-27205-02 (CoreCLR 4.6.27204.02, CoreFX 4.7.18.60501), 64bit RyuJIT

Runtime=Core  Toolchain=netcoreapp3.0  IterationTime=250.0000 ms
MaxIterationCount=20  MinIterationCount=15  WarmupCount=1

Method	Mean	Error	StdDev	Median	Min	Max	Gen 0/1k Op	Allocated Memory/Op
'GetCustomAttributes - Class: Hit (inherit)'	1,884.2 ns	33.686 ns	31.510 ns	1,877.4 ns	1,832.9 ns	1,948.8 ns	0.1204	384 B
'GetCustomAttributes - Class: Miss (inherit)'	463.8 ns	8.109 ns	7.585 ns	464.2 ns	450.6 ns	479.0 ns	-	-
'GetCustomAttributes - Class: Hit (no inherit)'	1,713.1 ns	17.682 ns	16.540 ns	1,706.6 ns	1,689.5 ns	1,738.3 ns	0.1211	384 B
'GetCustomAttributes - Class: Miss (no inherit)'	268.4 ns	3.372 ns	2.989 ns	268.3 ns	263.0 ns	272.5 ns	-	-
'GetCustomAttributes - Method Override: Hit (inherit)'	2,909.5 ns	56.034 ns	62.282 ns	2,899.0 ns	2,808.9 ns	3,019.5 ns	0.2057	656 B
'GetCustomAttributes - Method Override: Miss (inherit)'	1,796.4 ns	22.963 ns	21.480 ns	1,796.0 ns	1,751.6 ns	1,828.6 ns	0.1199	384 B
'GetCustomAttributes - Method Override: Hit (no inherit)'	1,765.0 ns	32.926 ns	32.338 ns	1,766.1 ns	1,710.7 ns	1,816.7 ns	0.1270	400 B
'GetCustomAttributes - Method Override: Miss (no inherit)'	1,726.6 ns	29.423 ns	27.522 ns	1,726.3 ns	1,675.9 ns	1,764.2 ns	0.1208	384 B
'GetCustomAttributes - Method Base: Hit (inherit)'	1,796.3 ns	37.857 ns	42.078 ns	1,794.1 ns	1,728.2 ns	1,900.2 ns	0.1180	384 B
'GetCustomAttributes - Method Base: Miss (inherit)'	311.5 ns	7.982 ns	7.466 ns	311.6 ns	302.5 ns	331.0 ns	-	-
'GetCustomAttributes - Method Base: Hit (no inherit)'	1,738.7 ns	31.928 ns	35.488 ns	1,732.8 ns	1,671.6 ns	1,819.6 ns	0.1163	384 B
'GetCustomAttributes - Method Base: Miss (no inherit)'	275.5 ns	3.779 ns	3.535 ns	276.0 ns	269.5 ns	280.3 ns	-	-

After

BenchmarkDotNet=v0.11.3.886-nightly, OS=Windows 10.0.17763.134 (1809/October2018Update/Redstone5)
Intel Core i7-7700HQ CPU 2.80GHz (Kaby Lake), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.0.100-preview-009820
  [Host]     : .NET Core 3.0.0-preview-27205-02 (CoreCLR 4.6.27204.02, CoreFX 4.7.18.60501), 64bit RyuJIT
  Job-FZOPMH : .NET Core ? (CoreCLR 4.6.27207.0, CoreFX 4.7.18.60402), 64bit RyuJIT

Runtime=Core  Toolchain=CoreRun  IterationTime=250.0000 ms
MaxIterationCount=20  MinIterationCount=15  WarmupCount=1

Method	Mean	Error	StdDev	Median	Min	Max	Gen 0/1k Op	Allocated Memory/Op
'GetCustomAttributes - Class: Hit (inherit)'	1,866.7 ns	36.295 ns	40.341 ns	1,860.8 ns	1,808.5 ns	1,971.1 ns	0.0656	224 B
'GetCustomAttributes - Class: Miss (inherit)'	468.7 ns	6.401 ns	5.988 ns	466.8 ns	459.8 ns	479.3 ns	-	-
'GetCustomAttributes - Class: Hit (no inherit)'	1,623.1 ns	31.073 ns	34.537 ns	1,608.3 ns	1,576.1 ns	1,698.8 ns	0.0652	224 B
'GetCustomAttributes - Class: Miss (no inherit)'	291.9 ns	5.004 ns	3.907 ns	292.5 ns	285.0 ns	298.0 ns	-	-
'GetCustomAttributes - Method Override: Hit (inherit)'	2,875.5 ns	52.543 ns	46.578 ns	2,878.0 ns	2,781.0 ns	2,947.3 ns	0.1479	496 B
'GetCustomAttributes - Method Override: Miss (inherit)'	1,713.2 ns	21.842 ns	20.431 ns	1,718.2 ns	1,676.8 ns	1,745.0 ns	0.0683	224 B
'GetCustomAttributes - Method Override: Hit (no inherit)'	1,669.4 ns	28.507 ns	26.665 ns	1,671.7 ns	1,628.2 ns	1,715.3 ns	0.0723	240 B
'GetCustomAttributes - Method Override: Miss (no inherit)'	1,682.3 ns	27.461 ns	25.687 ns	1,677.2 ns	1,647.7 ns	1,741.5 ns	0.0659	224 B
'GetCustomAttributes - Method Base: Hit (inherit)'	1,729.9 ns	33.490 ns	34.392 ns	1,728.0 ns	1,679.2 ns	1,787.5 ns	0.0693	224 B
'GetCustomAttributes - Method Base: Miss (inherit)'	325.6 ns	4.064 ns	3.802 ns	326.5 ns	317.6 ns	333.0 ns	-	-
'GetCustomAttributes - Method Base: Hit (no inherit)'	1,665.2 ns	32.926 ns	30.799 ns	1,666.3 ns	1,616.3 ns	1,716.5 ns	0.0659	224 B
'GetCustomAttributes - Method Base: Miss (no inherit)'	285.1 ns	5.131 ns	4.549 ns	285.0 ns	278.3 ns	293.5 ns	-	-

cc @jkotas @AviAvni @stephentoub

[jkotas]

Thanks

[AviAvni]

Thanks

[adamsitnik]

@NickCraver thank you!